A method and fluidized bed for applying color print to a substrate

Abstract: A specified and designated, non-primary color print (imaging) is applied to a substrate (paper) (43) by mixing at least first and second differently colored toner particles having substantially uniform physical characteristics, and introducing them in desired proportions into a fluidized bed (16). The toner powders are uniformly mixed together in the fluidized bed (as by using rotors (17) in addition to applying fluidizing air to the bed), and then a substantially uniform electrostatic charge (e.g. about +6.5-+8 kV D.C., which can be applied by blades on the rotor) is applied to the bed, and then the electrostatically charged mixture of toner particles is applied to the substrate (43), to image uniform non-primary color symbols on the substrate. The uniform physical characteristics of the powders are size (the vast majority of particles having a size between about 5 µm and about 25 µm), a resistivity of greater than 10¹² ohm-cm, and a flowability between a predefined minimum and maximum. When utilizing the fluidized bed as according to the invention, slight changes in chemical composition of the toners may be easily accommodated without change in the resulting uniform imaging.

Description: BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to a method of applying non-primary color print to a substrate and to a fluidised bed of toner particles for such a method. Colour toners are used in a variety of applications by both copiers and non-impact printers. Most of the toners commercially utilized are of the dual component type in which the toning system has larger magnetic carrier beads around which smaller pigmented toner particles attach themselves. The control of the toning system is achieved by a magnetic field associated with an applicator roller. The toner particles attach themselves to the magnetic beads by means of an electrostatic force generated by the electrostatic charge of the toner particle itself. It is that charge which also reacts to the force generated by the latent electrostatic image on the photoconductive cylinder, thus developing the image with toner. The electrostatic charge is generated by the triboelectric charging of the particle, that is the static charge generated by the toner particles rubbing and tumbling against the agitator, the sides of the toner sump, and each other. The triboelectric properties of the particles are determined by the composition of the toner particles themselves. Some particles are coated by surface additives to steer the particles toward the right polarity and the right magnitude of charge. However, any change in toner composition, even if slight, can totally change the charging characteristics, and thus the quality of the print produced. Using present toners and development systems it is extremely difficult to blend the toners uniformly because of the dependence of the tribo charging of the toner upon the composition of the toner used. To get two formulations to behave together as desired would be luck, rather than a reproducible event. Therefore some systems have introduced multi-color capability by the mixing of primary process colors on the paper. For example the Canon (RTM) CLC uses four toners, each imaging on a separate pass of the imaging cylinder, then mixing at the paper to form the desired color level. Of course this is expensive and has minimal throughput. The E-Print 1000 by Indigo attempts to do a similar job with liquid toner. In each case, though, four separate color toners must be utilized to create the desired color level with the complication of four developing stations. It is highly desirable to provide a simple charging and developing system which, by using specific primary colors of toners, would allow pre-blending before charging the system, to create specific levels of color for non-impact imaging of specified spot or highlight color applications. Arrangements in which two primary colour powders are mixed are disclosed in Patent Abstracts of Japan Vol.10 No. 374, (P-527) [2431] and JP-A-61 167960 and Xerox Disclosure Journal Vol.3 No. 5 p.291 of September/October 1978. It is particularly desirable to make the quality and uniformity of the color insensitive to slight chemical changes in the toner (from one batch to another). These desirable features are accomplished according to the present invention. The basic aspects of the present invention are to provide a fluidized bed of toner powders and to apply a uniform charge. This has been very difficult to accomplish in the past, and if the bed is not uniform, with a uniform charge, one color will have the tendency to deplete before the other, thereby changing the color on the printed substrate (e.g. paper). However this is avoided according to the present invention by making the various differently colored powders that are utilized in the fluidized bed so that they have substantially the same physical characteristics, such as resistivity, particle size, and flowability. Accordingly in one aspect the present invention provides a method of applying a designated, non-primary color print to a substrate, comprising the steps of: mixing at least first and second differently colored toner powder having particles, applying an electrostatic charge to the toner particles and applying the electrostatically charged toner particles to a latent image in order to create a non-primary color image on the substrate, characterised in that before the first and second toner powders are charged they are introduced into a fluidized bed in desired proportions, the particles are mixed and the charge applied to the mixture in the fluidized bed, and the first and second toner powders have substantially the same particle size, resistivity and flowability such that they can be charged uniformly. The mixing is typically practiced utilizing primary color toners as the differently colored toner powder, and two or more different powders may readily be utilized. Preferably the vast majority of particles making up the toner powders have a size between about 5 µm and about 25 µm (e.g. between about 10-15 µm). The resistivity of the toner powders is preferably greater than about 10 12 ohm-cm. The toner powders have flowability between a predefined minimum and maximum, the minimum being established by empirical means, and the maximum by the flowability that would make handling of the powder in mechanical systems too unreliable. The method is practiced utilizing a fluidized bed apparatus such as disclosed in european application No. 91122358.4 filed on January 8, 1991, [published as EP-A-0 494 454]. In that system, one or more rotors with a plurality of radially extending sharp points are mounted within the fluidized bed, serving to mix the particles together and also to apply a high, uniform charge to the particles. Typically a sufficiently high D.C. voltage is applied, with sufficient concentration, to breakdown molecules in the vicinity of the source application into individual ionic species, e.g. into positive species, comprising H + (H 2 O) n , where n = 1, 2, ... 6. Normally this is accomplished by applying approximately + 6.5- + 8 kV potential, producing a charge sufficient to associate charges of greater (on the average) than 20 microcoulombs/gram with the individual toner particles. The invention also provides a fluidized bed of toner particles comprising a first toner powder having particles with a charge, a second toner powder having particles with a charge, characterised in that the first and second toner particles are of different colors while their resistivity, flowability and particle size are substantially the same such that the charge on each particle is also the same. The average predetermined charge of the particles is greater than 20 microcoulombs/gram, the vast majority of the particles have a particle size of between about 5-25 µm, and the particles have a resistivity of greater than 10 12 ohm/cm. The colors typically may be primary colors, and a third toner powder or more of a primary color different than the first and second colors is also preferably provided. It is the primary object of the present invention to provide a simple and effective method of color imaging for copiers, non-impact printers, or the like, utilizing a system that is insensitive to slight changes in chemical composition, i.e. small changes in chemical composition of the toners being added to the system not making a change in the uniformity of the imaging produced utilizing the toners, such as by utilizing a Moore MIDAX 300 system. (Moore and MIDAX are registered Trade Marks). This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic side view of an exemplary system, including the fluidized bed according to the invention, for practicing an exemplary method according to the invention; and FIGURE 2 is a plan view of a substrate with uniform non-black, non-primary color print produced according to the invention. DETAILED DESCRIPTION OF THE DRAWINGS An exemplary system for applying a specified, non-primary color print to a substrate is shown in FIGURE 1. The basic apparatus illustrated in FIGURE 1 is the same as that illustrated in european application number 91122358.4 filed January 8, 1991, published as EP-A-0 494 454 except that it has been determined that there is no need for the electrical vibrator, it being possible to provide sufficient fluidizing action with the introduced air. The fluidized bed container 10 in FIGURE 1 has an input tube 11 for fluidizing air, a distribution plenum 12, and a semi-pervious plate 13, such as a piece of porous stainless steel typically used in filtration applications in industry. The plate 13 typically has an average opening of 0.2 micrometers through its pores. The container defining the fluidized bed 10 is typically made up of an insulating polymer such as Delrin. Located within the container 10 are rotors 14 having a plurality of discharge points 17 extending radially outwardly therefrom, and supplied with a charge from the source 15, such as a +6.5-+8 kV D.C. source. Within the container 10 are the fluidized toner particles in fluidized bed 16. According to the invention there are at least two different designated colored toner particles making up bed 16, typically two or three primary colored toners in predetermined proportions depending upon final specified and designated, non-primary color symbols desirably printed. Each color has a characteristic hue, chroma and intensity. Hue depends on the proportion of the primary colored toners added, e.g., yellow - magenta - cyan. Chroma is essentially the strength of a color and is governed by the amount of toner per square unit area applied to the substrate. Chroma can be adjusted by controlling the applied amount of clear toner. Intensity is the blackness or whiteness of the color. Intensity is controlled by adding black or white toner and depends on the brightness of the substrate. Toner is added to the bed 16 in response to sensing by level sensor 19, utilizing the dispenser 20, while the rollers 31, 33 and 35 (typically made of plain cold roll steel plated with hard chrome and polished) are utilized to apply the toner particles to the latent electrostatic image being carried on the image cylinder 41. This image is transferred to the substrate 43 (typically, a moving web of paper), under the applied force from the impression cylinder 42 which is in opposition to the image cylinder 41, as described in european application number 91122358.4 filed January 8, 1991, published as EP-A-0 494 454. In order to achieve the desired results according to the invention, it is necessary that the physical properties of the different toners added to the bed 16 be uniform. The most important physical characteristics are particle size, resistivity, and flowability. The particle size is preferably such that the vast majority of the particles are between about 5-25 µm (typically about 10-15 µm). For example the average size of the particles may be 15 µm, which have been classified to reduce particles of less than 5 µm to under 10% of the total distribution. Other classification to remove the majority of the particles above 25 µm may be necessary under select circumstances. The resistivity of the particles of both the first, second, or subsequent differently colored toner particles are greater than about 10 12 ohm-cm, and the flowabilities between a predefined minimum and maximum. The minimum flowability will be determined empirically for different situations, while the maximum is limited by the ability of mechanical systems to reliably handle the powders. The charge applied to the particles in the bed 16 by the plurality of discharge points 17 extending outwardly from the rotors 14 is of a sufficiently high D.C. voltage, with sufficient concentration, to breakdown molecules in the vicinity of the blades 17 into individual ionic species. For example the molecules are broken down into positive species comprising H + (H 2 O) n' where n = 1, 2, ... 6. Typically the source 15 has a D.C. voltage of between about +6.5-+8 kV, and the charge supplied is sufficient to associate average charges of greater than 20 microcoulombs/gram with the individual toner particles. One possible example of creating a specific color from two primary toners in an electrostatic fluidized bed will now be described. Example The specific color to be produced is a dark purple color, identified as Pantone PMS color 259. Creation of the color is by mixing in the fluidized bed 16 a typical magenta colored primary and a typical cyan colored primary. The cyan toner component is made as follows. Blend approximately 4% copper pthalocyanine pigment into a polyester resin matrix (e.g. ATLAC (RTM) 382E by Reichold (RTM)). This compound is jet-milled to an average particle size of 15 µm and classified to reduce particles of less than 5 µm to under 10% of the total distribution. This powder is postblended with 0.75% by weight of the treated fumed silica flowing agent (e.g. Cabot (RTM) TS-530 or equivalent) in a high speed mixer such as a Henschell (RTM) or an Omni (RTM). The magenta component starts with a blend of approximately 4% Hasta-Perm (RTM) PINK-E pigment (Harshaw Chemical) in the same polyester resin used in the cyan blend above. All processing of the toner is done in the same sizing, grinding, classification, and post blending steps as those described above. Both primary components are then blended together, either before introduction into the electrostatic fluidized bed 16, or actually mixed into the bed 16. For the purple color described above, the typical ratio would be very near three parts of magenta toner to one part of the cyan toner. In a second example, five separate intensities of blue, including PMS 290, were produced by incrementally diluting a cyan primary with a white toner. The cyan component of the blend is made as described above. The white component starts with a blend of approximately 4.5% titanium dioxide pigment in the same polyester resin used in the cyan blend. Both the white and cyan primary components are then blended together either before introduction into the electrostatic fluidized bed 16, or actually mixed into the bed. Diluting in a serial fashion, a ratio of 1 part white to 2.5 parts cyan will produce a PMS 299 shade. Using this blend as a new primary shade, a second dilution of 1 part white to 2.5 parts of the blend produced very nearly a PMS 298 shade. Successive further dilutions in the same ratio yielded a PMS 297 shade, a PMS 290, and a final shade lighter than PMS 290 not found in the listed color sample. The multi-roller electrostatic toning unit is operated then in the normal mode as described in european application number 91122358.4. published as EP-A-0 494 454. Approximately a +6.5 to +8.0 kV potential is applied to the in-bed corona arrays, 14, 17 which creates a large population of positive ionic species. These ions attach to both the cyan and magenta toner particles in equal proportions. This mixed blend of toners transfers via electrical field forces from roller 31 to roller 33, ultimately onto the image cylinder 41. From there, the mix of cyan and magenta toners is transferred to the paper 43 and fused to the paper infra-red heating. Through the process, the individual toner particles lose their individual color properties because of their small size, the amount of mixing and motion going on, and the final combination together into a singular fused image on the paper. What results is the desired dark purple image symbols 47 on the paper 48 (see FIGURE 2). This same principle was also demonstrated in the serial dilution in the shades for the light blue PMS series. The fluidized bed 16 is insensitive to changes in chemical composition of the toner that is being introduced, unlike prior art systems. Thus a completely uniformly colored print of symbols 47 is provided. It will thus be seen that according to the present invention a simple, effective method (and fluidized bed) for printing a substrate, such as paper, with toner, useful for both copier and non-impact printing (such as electrostatic printing) is provided. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, as specified in the appended claims.

Claims: A method of applying a designated, non-primary color print to a substrate, comprising the steps of: mixing at least first and second differently colored toner powder having particles, applying an electrostatic charge to the toner particles and applying the electrostatically charged toner particles to a latent image in order to create a non-primary color image on the substrate, characterised in that before the first and second toner powders are charged they are introduced into a fluidized bed (16) in desired proportions, the particles are mixed and the charge applied to the mixture in the fluidized bed, and the first and second toner powders have substantially the same particle size, resistivity and flowability such that they can be charged uniformly. A method according to claim 1 characterised in that the mixing includes subjecting the toner powders to rotating mechanical structures (14) while fluidizing them. A method according to claim 1 or claim 2 characterised in that the first and second differently colored toner powders are primary colored toners. A method according to any of claims 1 to 3 characterised in that the vast majority of particles making up the toner powders have a size of between substantially 5 µm and substantially 25 µm, preferably between 10 µm and 15 µm. A method according to any of claims 1 to 4 characterised in that toner powders have a resistivity of greater than substantially 10 12 ohm-cm. A method according to any of claims 1 to 5 characterised in that the particles are changed by a high voltage source (15) sufficiently to break down molecules in the vicinity of the source discharge points into individual ionic species. A method according to claim 6 characterised in that the charge is applied so as to break down the molecules into positive species comprising H + (H 2 O) n , where n = 1, 2, ...6. A method according to any of claims 1 to 7 characterised in that the charge is applied by applying approximately + 6.5 - + 8 kV potential to voltage discharge points (17) in the fluidized bed. A method according to any of claims 1 to 8 characterised in that the average charge of the individual toner particles is greater than 20 microcoulombs/gram. A method according to any of claims 1 to 9 characterised in that a post blended flowing agent is added to the mixture of toner particles. A method according to any of claims 1 to 10 characterised by the inclusion of at least a third toner powder of a primary color different from the first and second colors but having substantially the same resistivity and particle size range. A fluidized bed (16) of toner particles comprising a first toner powder having particles with a charge, a second toner powder having particles with a charge, characterised in that the first and second toner particles are of different colors while their resistivity, flowability and particle size are substantially the same such that the charge on each particle is also substantially the same. A fluidized bed according to claim 12 characterised in that the average predetermined charge of the particles is greater than 20 microcoulombs per gram, the vast majority of the particles have a particle size of between 5 and 25 µm and the particles have a resistivity of greater than substantially 10 12 ohm-cm. A fluidized bed according to claim 12 or claim 13 in which the first and second colors are primary colors and characterised by the inclusion of at least a third toner powder of a primary color different from the first and second colors but having substantially the same resistivity and particle size range.

Inventors: CHRISTY ORRIN D, MATHEIS MARK A, PICKETT JOHN E, SWANSON LEO, CHRISTY, ORRIN D., MATHEIS, MARK A., PICKETT, JOHN E., SWANSON, LEO
Applicants: MOORE BUSINESS FORMS INC, MOORE BUSINESS FORMS, INC.
Application year: 1994
Main classification: G03G15
IPC codes: G03G9, G03G13, G03G15